1. Field of the Invention
The present invention relates to a surface acoustic wave device in which interdigital finger electrodes are provided on a piezoelectric substrate, and a piezoelectric substrate for the surface acoustic device.
2. Prior Art
In these years, mobile communication terminal equipments inclusive of portable phones has been rapidly popularized. In view of portability, it is desirable for such terminal equipments particularly to be compact and lightweight.
To achieve size and weight reductions for the terminal equipments, it is also necessary for electronic elements applied thereto to be compact and lightweight. For this reason, a surface acoustic wave device, i.e. a surface acoustic wave filter, which is advantageous to size and weight reductions has often been applied as a filter in high and intermediate frequency elements of the terminal equipments.
The surface acoustic wave device is of a structure in which an interdigital finger electrode is formed on a main surface of a piezoelectric substrate for exciting, receiving, reflecting or propagating surface acoustic waves. Important characteristics as to the piezoelectric substrate applied to this surface acoustic wave device are the surface wave velocity of the surface acoustic wave (thereinafter, sometime referred to as SAW velocity), the temperature coefficient of frequency (TCF) pertain to a center frequency in the case of filters or a resonance frequency in the case of a resonators, and an electromechanical coupling coefficient (K2).
FIG. 1 shows a table listed various compositions of piezoelectric substrates which have typically been applied to the surface acoustic wave device so far. The well-known piezoelectric substrates shown in FIG. 1 can be broadly classified into two groups. One group includes 128LN, 64LN and 36LT which have fast SAW velocity and large electromechanical coupling coefficient, and another group includes LT112 and ST quartz which have relatively slow SAW velocity and small electromechanical coupling coefficient. Among these substrates, 128LN, 64LN and 36LT which are piezoelectric substrates having high SAW velocity and large electromechanical coupling coefficient are applied to surface acoustic wave filters in high frequency elements of the terminal equipments, while LT112 and ST quartz which are piezoelectric substrates having low SAW velocity and small electromechanical coupling coefficient are applied to surface acoustic wave filters in intermediate frequency elements of the terminal equipments. Because, in the surface acoustic wave filter, its center frequency is approximately directly proportional to the SAW velocity of the applied piezoelectric substrate and is approximately inversely proportional to the electrode finger width of the interdigital finger electrode formed on the piezoelectric substrate.
Therefore, when such a filter is constructed to apply to a high frequency circuit element, the substrate having high SAW velocity would be desired. In addition, since the filter applied to the high frequency circuit element of the terminal equipments is required to be a wideband filter whose passbandwidth is 20 MHz or more, it is also necessary for its electromechanical coupling coefficient to be large.
On one hand, the frequency band ranging from 70 to 300 MHz is used as the intermediate frequency for the mobile terminal equipments. In case that a filter which has a center frequency within this frequency band is constructed by a surface acoustic wave device, if the piezoelectric substrate having high SAW velocity is applied, it needs to considerably increase the electrode finger width formed on the substrate in proportion to the decreased amount of the center frequency, in comparison with that of the filter applied to the high frequency circuit. This causes a problem that the size of a surface acoustic wave device itself becomes large. Thus, LT112 or ST quartz having slow SAW velocity is generally applied to surface acoustic wave filters for intermediate frequency circuit elements. In particular, ST quartz is a desirable piezoelectric substrate material due to zero of its first order temperature coefficient of frequency. ST quartz can be constructed only for filters having narrow passband due to small electromechanical coupling coefficient of ST quartz. In the past, such small electromechanical coupling coefficient did not very become an issue because the function of intermediate frequency filters was to make only one narrow channel signal pass through.
However, in recent years, for the reason of effective utilization of frequency resources and adaptability to digital data communications, a digital mobile communication system has been developed, made practicable and rapidly popularized. The passbandwidth of this digital mobile communication system becomes extremely wide band, such as several hundred KHz to several MHz. In case that the intermediate frequency filter having such wide band is constructed by a surface acoustic wave device, it is difficult to be implemented in ST quartz. Further, when promoting size reduction of the mobile terminal equipment to enhance its portability, the mounting area of the surface acoustic filter for intermediate frequency is required to be reduced in size. However, ST quartz and LT112 which were described as materials suited to the surface acoustic filter for intermediate frequency so far have some limitation on size reduction due to the fact that their SAW velocity is over 3000 m/sec.
It is the object of the present invention to provide a piezoelectric substrate for a surface acoustic wave device which has high electromechanical coupling coefficient effective to widen passband and low SAW velocity effective to reduce in size of the surface acoustic wave device, and a wide band, compact surface acoustic wave device using the same.
To solve the problem described above, the present invention puts forward to use a piezoelectric substrate characterized in that the piezoelectric substrate comprises a single crystal belonging to a point group 32 and having Ca3Ga2Ge4O14 type crystal structure, and its basic component is comprised of La, Sr, Ta, Ga and O and is represented by the chemical formula La3xe2x88x92xSrxTa0.5+0.5xGa0.5xe2x88x920.5xO14. In this case, the composition ratio of Sr is preferably in the range of 0 less than xxe2x89xa60.15, and more preferably in the range of 0.07 less than xxe2x89xa60.08. According to a basic concept of the present invention, a possibility can be developed which a piezoelectric substrate having high electromechanical coupling coefficient and low SAW velocity effective is obtained by using the single crystal having the aforementioned composition as a piezoelectric substrate for a substrate acoustic wave device.
The present invention also provides a surface acoustic wave device using that in which an interdigital finger electrode is formed in one main surface of the aforementioned piezoelectric substrate.
According to one preferable aspect of the present invention, when a cut angle of the substrate cut out of the single crystal and a propagation direction of surface acoustic waves are represented in terms of Euler angles (xcfx86, xcex8, "psgr"), these angles lie within the area 1: xcfx86=xe2x88x922.5xc2x0 to 2.5xc2x0, xcfx86=60xc2x0 to 120xc2x0, "psgr"=xe2x88x9240xc2x0 to 40xc2x0.
According to another aspect of the present invention, these angles lie within the area 2: xcfx86=xe2x88x922.5xc2x0 to 2.5xc2x0, xcex8=120xc2x0 to 160xc2x0, "psgr"=10xc2x0 to 50xc2x0. In still another aspect of the present invention, these angles lie within the area 3: xcfx86=xe2x88x922.5xc2x0 to 2.5xc2x0, xcex8=120xc2x0 to 160xc2x0, "psgr"=xe2x88x9250xc2x0 to xe2x88x9210=20 , the area 4: xcfx86=2.5xc2x0 to 7.5xc2x0, xcex8=60xc2x0 to 120xc2x0, "psgr"=xe2x88x9230xc2x0 to 30xc2x0, the area 5: xcfx86=2.5xc2x0 to 7.5xc2x0, xcex8=120xc2x0 to 160xc2x0, "psgr"=0xc2x0 to 50xc2x0, the area 6: xcfx86=2.5xc2x0 to 7.5xc2x0, xcex8=120xc2x0 to 160xc2x0, "psgr"=xe2x88x9240xc2x0 to xe2x88x9210xc2x0, or the area 7: xcfx86=27.5xc2x0 to 32.5xc2x0, xcex8=10xc2x0 to 160xc2x0, "psgr"=xe2x88x92400xc2x0 to 30xc2x0.
When the aforementioned angles lie within the area 1, the SAW velocity of the substrate becomes less than or equal to 3000 m/sec and is presented in a lower value compared with that of ST quartz. In addition, there exists a combination of (xcfx86, xcex8, "psgr") in which the electromechanical coupling coefficient of the substrate is presented in a sufficiently high value, i.e. more than or equal to 0.2%. In the area 1, it is preferable to arrange in the range of xcfx86=xe2x88x922.5xc2x0 to 2.5xc2x0, xcex8=80xc2x0 to 115xc2x0, "psgr"=xe2x88x9220 xc2x0 to 20xc2x0. Within this preferable area, it has been found that on top of which the SAW velocity of the substrate becomes less than or equal to 3000 m/sec, the electromechanical coupling coefficient of the substrate becomes a sufficiently high value, i.e. more than or equal to 0.3%.
When the angles lie within the area 2, the SAW velocity of the substrate is also represented in a lower value compared with that of ST quartz, i.e. less than or equal to 3000 m/sec, and the electromechanical coupling coefficient of the substrate is represented in a sufficiently high value, i.e. more than or equal to 0.2%. In the area 2, it is preferable to arrange in the range of xcfx86=xe2x88x922.5xc2x0 to 2.5xc2x0, xcex8=130xc2x0 to 150xc2x0, "psgr"=15xc2x0 to 30xc2x0. Within this preferable area, on top of which the SAW velocity of the substrate becomes less than or equal to 3000 m/sec, the electromechanical coupling coefficient of the substrate becomes a sufficiently high value compared with that of ST quartz, i.e. more than or equal to 0.3%. A similar result can be obtained in the area 3. In the area 3, it is preferable to arrange in the range of xcfx86=xe2x88x922.5xc2x0 to 2.5xc2x0, xcex8=130xc2x0 to 150xc2x0, "psgr"=xe2x88x9230xc2x0 to xe2x88x9215xc2x0 because of the same reason as that of the area 2. In the area 4 through 7, similar effects as those of the areas described above can also be obtained. Besides, in the area 7, it is preferable to arrange in the range of xcfx86=27.5xc2x0 to 32.5xc2x0, xcex8=120xc2x0 to 140xc2x0, "psgr"=5xc2x0 to 15xc2x0, or xcfx86=27.5xc2x0 to 32.5xc2x0, xcex8=40xc2x0 to 60xc2x0, "psgr"=5xc2x0 to 15xc2x0. In these preferable areas, the electromechanical coupling coefficient is presented in a significantly high value, i.e. more than or equal to 0.5%. As described above, according to the present invention, the electromechanical coupling coefficient of the substrate can be made large to achieve a wide band surface acoustic wave device. The SAW velocity of the substrate can also be slowed to reduce in size the surface acoustic wave device.
Since the single crystal of the chemical formula La3xe2x88x92xSrxTa0.5+0.5xGa5.5xe2x88x920.5xO14 is a trigonal system, mutually equivalent combinations of Euler angles exist due to crystal symmetry. Thus, in the present invention, similar effects can be obtained not only from the angles shown in the following embodiments, but also from the angles which are crystallographically equivalent to these embodiments. As example of the equivalent combinations, for example, a combination of (0xc2x0, 90xc2x0, 20xc2x0) on angles (xcfx86, xcex8, "psgr") in the area 1 is equivalent to a combination of (0xc2x0, 90xc2x0, xe2x88x9220xc2x0), or a combination of (240xc2x0, 90xc2x0, xc2x120xc2x0) and a combination of (120xc2x0, 90xc2x0, xc2x120xc2x0). A combination of (0xc2x0, 90xc2x0, 20xc2x0) included in the area 1 is also equivalent to a combination of (240xc2x0, 90xc2x0, xc2x120xc2x0), and a combination of (360xc2x0, 90xc2x0, xc2x120xc2x0). Further a combination of (30xc2x0, 90xc2x0, 0xc2x0) included in the area 3 is equivalent to a combination of (90xc2x0, 90xc2x0, 0xc2x0). Therefore, it is to be understood that the present invention includes equivalent range based on such symmetric property of the crystal.
The single crystal of La3xe2x88x92xSrxTa0.5+0.5xGa5.5xe2x88x920.5xO14 may have oxygen defect, and may includes inevitable impurities, such as Al, Zr, Fe, Ce, Nd, La, Pt, Ca, and the like.